The development of the atheromatous plaque is largely dependent on vascular smooth muscle cell proliferation and production of biologically active compounds such as cytokines and growth factors. Cytokines such as IL-1 derived from blood vessel wall may contribute to regional defense or pathology. Neutralization of the effects mediated by IL-1 by a receptor antagonist specific for IL-1 alpha and IL-1 beta has been shown to reduce the possible pathologic consequences induced by IL-1 in the regional environment. The effect of human recombinant interleukin-1 receptor antagonist (hrIL-1ra), a new member of the IL-1 family, has been assessed on modulating vascular smooth muscle cell (VSMC) proliferation in the rat. A significant dose- and time-dependent reduction of DNA synthesis was observed when hrIL-1ra was added to the cell cultures. The maximum inhibitory effect was seen using IL-1ra at a concentration of 250 ng/ml and after 48 h incubation with cultured vascular smooth muscle cells. Furthermore, hrIL-1ra inhibited VSMC growth in the presence of exogenous mitogenic doses of IL-1 alpha. The addition of indomethacin to the cultures did not modify the inhibitory events. These data suggest a possible pharmacologic role for IL-1ra in inhibiting VSMC proliferation by possibly interfering with the autocrine regulatory pathway of IL-1.